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Secondary mirror woes

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Hello all

Ive allways done the secondary with a 35 mm film canister and hole in lid method and allways doubted wether its bang on centre,trouble being theres just a bit too much space around the mirror to gauge with the eye.

So today i made a sight tube which is alot longer and just leaves enough space around the mirror so the eye can gauge if it is centre under the draw tube.

It is out slightly as i thought but the problem is, its not out left or right ,towards primary and back as this would be a matter of the centre secondary screw ,its out up and down.

As the three set srews on the secondary adjust the tilt,i just assumed it must be the spider veins holding the secondary mirror in place not being central.

The spider has been checked and is holding the secondary mirror bang on centre in the scope

Ive heard about drawtubes not being square,could this be the problem? and how to correct it ive heard the word "shimming" used to correct this but im clueless how to go on about it...if this is the problem !

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